Re: [w3c/payment-request] remove currencySystem member (#694)

Thanks everyone for the productive discussion here. I want to give a summary of where we are so far (and perhaps we should add the following to the spec as a note, as @ianbjacobs suggested). 

> User agents implementing Payment Request enforce ISO 4217's 3-letter codes format via ECMAScript’s “isWellFormedCurrencyCode” algorithm. When a code does not adhere to the format, a `RangeError` is thrown. 
> 
> Current implementations of the Payment Request API will therefore allow the use of well-formed currency codes that are not part of the official ISO 4217 list (e.g., XBT, XRP, etc.). If the provided code is a currency that the browser knows how to display, then an implementation will generally display the appropriate currency symbol in the user interface (e.g., "USD" is shown as "$", "GBP" is "£", and the non-standard "XBT" could be shown as "Ƀ"). When a code can't be matched, the specification recommend browsers show a [scarab](https://en.wikipedia.org/wiki/Currency_sign_(typography))  "¤". 
> 
> Given the rise of digital currencies in recent years, ISO 4217's format has been found wanting. For example, does "BTC" refer to Bitcoin or to a future Bhutan currency according to the 4217 naming rules? To overcome the limitations with ISO 4217 with respect to digital currencies, ISO is in the early stages of evolving ISO 4217. At the time of publication, it remains unclear what form this evolution will take, or even the timeframe in which the work will be completed. The W3C Web Payments Working Group is liaising  with ISO during this evolutionary process to ensure digital currencies are well supported both in future versions of  ISO 4217 and in future revisions of the Payment Request API. 

@adrianhopebailie wdty? I can send the above as PR and we can iterate. 

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